1. Field of the Invention
The present invention relates generally to the field of electronic filters. In particular, the present invention relates to a transversal filter design that, when used in an echo canceller, gives improved echo cancellation of a transhybrid response having well behaved portions. While the present invention has numerous other applications, it will be described in connection with its use in an integrated services digital network (ISDN) telecommunication system.
2. Background Information
The telephone system has expanded to such a degree that today it is possible for almost anybody to have almost instantaneous communications with almost anybody else almost anywhere in the world. Since the phone system is so ubiquitous, it provides a desirable communication medium for more than just voice-to-voice communication. Additional services such as facsimile transfers, telemail, electronic funds transfer, video data, computers links, and alarm systems benefit from the use of the telephone system.
Using the existing telephone system as a communication channel for the additional services involves more than just connecting a machine to a phone line. For example, the existing phone system wa only designed to transmit analog signals with a bandwidth of about 300-3500 hertz, sufficient for voice communication but not optimum for the additional services which generally transmit data having significantly higher frequency components. The result is that the signals of the additional services distort as they propagate.
Aggravating the problem of using the present system is the fact that only a single twisted pair of wires interconnect a subscriber (user) with a central office. This means that the subscriber's transmitter (mouthpiece) and receiver (earpiece) share the same two wires. While this is a low cost method of connecting subscribers to a central office, because full duplex (simultaneous two-way) operation is desired cancellation of a subscriber's transmission from his own receiver is required. This is understood by considering that if a first and second subscriber both transmit at the same time, the second subscriber's signal attenuates as it travels toward the first subscriber. If the first subscriber does not cancel out of his receiver his own transmission he will hear only his own transmission and not the desired, attenuated second subscriber's signal.
Full duplex operation on two wires therefore requires that a transmitted signal must be reduced sufficiently from the transmitter's own receiver to allow reception of an incoming signal. While readily accomplishable with voice communication, it is much harder to do with the additional services because those services are more sensitive to incompletely cancelled signals.
Exacerbating the problem of increased sensitivity to incompletely cancelled signals are the numerous wire taps, wire gauge changes, and switching networks which cause signal "reflections" in the present phone system. These reflections can be picked up by the transmitter's receiver and, if not handled properly, could be mistaken for a signal from another subscriber.
Even further complicating the use of the present phone system as a communication medium is that the present phone lines have widely variable transmission line characteristics. This creates a problem because any mismatch between the telephone line and the service using the phone line causes an "incident" signal that is reflected into the receiver.
The effects of insufficient echo cancellation, line taps, mismatches, and high frequency are more serious when using the additional services than with just voice communications. To assist in reducing these and other problems, the Accredited Standards Committee on Telecommunications, T1, submitted to the American National Standards Institute a standard for integrated services digital networking, ANSI T1.601-1988, entitled "Integrated Services Digital Network (ISDN)--Basic Access Interface for Use on Metallic Loops for Application on the Network Side of the NT (Layer 1 Specification)." This document describes a minimal set of requirements and protocols for satisfactory communication between subscribers when using the additional services with full duplex operation on the existing single twisted wire pair phone line.
As previously indicated, for acceptable full duplex operation when using the existing single twisted pair phone lines with the additional services the transmitted information must be more fully cancelled despite the increased reflections and the incident signal. Adding together both the reflections and the incident signal one obtains the "transhybrid response." The transhybrid response is then the echo response verses time caused by a transmitted signal.
A signal transmitted on a phone line may become attenuated by as much as 40 db (100 times), making a 5/6 volt transmit signal only about 0.0085 volts at the receiver. Since an echo may be almost as large as a transmit signal, or up to about 2.5 volts, a receiver may be required to detect a 0.0085 volt signal riding on a 2.5 volt echo. Reduction of the echo amplitude to an acceptable amount is the job of the echo canceller.
While the echo canceller must reduce the echo to an acceptable amount (roughly a 60 db reduction), the actual transhybrid response is unknown until the lines connecting the subscribers is established, something that does not occur until a call is answered. Therefore, acceptable methods of echo cancelling must be adaptive, i.e. they must adjust to the line conditions existing at the time of the call.
One commonly used method of echo cancelling was proposed by Kurt H. Mueller in "A New Digital Echo Canceller for Two-Wire Full-Duplex Data Transmission" found in the IEEE Transactions on Communications, September 1976, at pp. 956-962. Mr. Mueller proposed using a transversal filter to eliminate the transhybrid response by adjusting the filter's output to cancel the response.
A transversal filter, as used by Mr. Mueller and others, is an electronic device having a storage unit that sequentially stores the data transmitted in each baud period, i.e. a shift register able to store as many data bits as is transmitted. Each baud period's data stored in the shift register is sent to a multiplier whose gain is selected so that each baud period's data, when multiplied by the gain, equals the transhybrid response caused by that data. The summation of all gain multiplied data transmissions is then subtracted from the echo, ideally eliminating it. Each multiplier circuit output is called a tap.
While conceptually it is easy to visualize that a transversal filter based echo canceller could completely cancel the transhybrid response, an infinite number of taps corresponding to an infinite number of prior baud period's transmissions would be required. Since it is impossible to implement an infinite number of taps, in an actual echo canceller the number of taps is limited. This limitation implies that some of the transhybrid response is not cancelled and thus remains as a residual echo.
The assignees of the present invention have found from experimentation on actual telephone lines and from software simulation that latter parts of the transhybrid response when meeting the requirements of ANSI T1.601-1988 are "well behaved" and of an exponentially decaying nature. Well-behaved describes a function similar to one having a dominate single pole with a long time constant, which results in a function having a portion which changes slowly in an exponential manner.
It is therefore useful and desirable to reduce the residual echo from a transhybrid based echo canceller with a given number of taps. Likewise, it is useful and desirable to produce a similar amount of residual echo from a transhybrid based echo canceller having a smaller number of taps.